The Test Sample

What is being tested?

Protein C and protein S are two proteins in the blood that help regulate blood clot formation. Two separate tests for these proteins are often performed together as part of the investigation of a possible excessive clotting disorder. The tests measure the amount of each protein and evaluate whether they are performing their proper functions.

Normally, when a body tissue or blood vessel wall is injured, a process called hemostasis begins to form a plug at the injury site to help stop the bleeding. Small cell fragments called platelets adhere to and aggregate at the site and a coagulation cascade begins as proteins called coagulation factors are activated one after the other. Eventually, a stable clot forms, preventing additional blood loss and remaining in place until the injured area has healed. The clot is then broken down when it is no longer needed. There must be an adequate amount of platelets and clotting factors and each must function normally in order for a stable clot to form.

Proteins C and protein S work together to help control blood clot formation. They inactivate specific coagulation factors (factors V and VIII) that are required to generate and form blood clots. This has the net effect of slowing down clot formation, much like brakes slow a speeding car. However, if there is not enough protein C or S or they are not functioning normally, clot formation can go unchecked, possibly leading to excessive clotting. These conditions can range from mild to severe.

Deficient or dysfunctional protein C or protein S may be due to an underlying condition (acquired), such as liver disease, kidney disease, severe infections or cancer, or can be inherited, passed from parent to child. About 1 out of every 300 people has one normal gene and one abnormal gene (heterozygous) for protein C deficiency and about 1 in 20,000 people have protein S or C deficiencies that lead to symptoms.

There are two types of inherited protein C deficiencies:

Type 1 is related to quantity.

Type 2 is related to abnormal function and is less common than Type 1.

Protein S exists in two forms in the blood: free and bound to another protein, but only the free protein S is available to combine with protein C. There are three types of inherited protein S deficiencies:

Type 1 deficiency is due to an insufficient quantity.

Type 2 is due to abnormal function.

Type 3 is due to low free protein S levels, though total protein S levels are normal.

Two types of tests may be used to evaluate protein C and protein S:

Functional tests for protein C and protein S measure their activity and evaluate their ability to regulate and slow blood clotting. Decreased activity may be due to a decreased amount of protein C or S or, more rarely, due to dysfunctional protein C or S.

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This article was last reviewed on March 18, 2015. | This article was last modified on October 30, 2015.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.